Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. Such multimedia projection systems typically receive from a personal computer (“PC”) analog video signals representing still, partial-, or full-motion display images that are converted into digital video signals for controlling a digitally driven image-forming device, such as a micro-electromechanical display device (“MDD”), a common type of which is a digital micromirror device. An example of a popular MDD-based multimedia projector is the model LP420 manufactured by In Focus Corporation, Wilsonville, Ore., the assignee of this application.
Significant effort has been invested into developing multimedia projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected images having adequate brightness are difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
FIG. 1, shows a typical prior art multimedia projector 30 including a light source 32 that propagates polychromatic light along an optical path 34. Light source 32 generates intense light by employing an arc lamp 36 and an elliptical reflector 38. Optical path 34 includes a condenser lens 40, a color wheel 42, a light integrating tunnel 44, a fold mirror 46, a relay lens 48, an MDD 50, and a projection lens 52. One or two field lenses (not shown) typically follow light integrating tunnel 72. The optical components are held together by an optical frame 54 that is enclosed within a projector housing (not shown). A display controller 56 receives color image data from a PC 58 and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to MDD 50 in proper synchronism with the rotating angular position of color wheel 42. A power supply 60 is electrically connected to light source 32 and display controller 56 and also powers a cooling fan 62 and a free running DC motor 64 that rotates color wheel 42. Display controller 56 controls MDD 50 such that light propagating from relay lens 48 is selectively reflected by MDD pixel mirrors either toward projection lens 52 or toward a light-absorbing surface 66 mounted on or near optical frame 54. Color wheel synchronization is achieved by an appropriate sensor coupled to color wheel 42 or by employing a color selective light sensor 68 to detect the time period during which a predetermined color filter segment is in optical path 34.
To increase projected image brightness and uniformity, an input aperture 70 of light integrating tunnel 44 collects light exiting color wheel 42 and homogenizes the light during propagation through tunnel 44 to an output aperture 72. The uniformly bright rectangular light bundle exiting output aperture 72 propagates through the field lenses, reflects off fold mirror 46, and is imaged by relay lens 48 onto MDD 50. Unfortunately, because of the oblique illumination angle of MDD 50, the bright image of output aperture 72 typically overfills at least a portion of MDD 50 resulting in reduced brightness of the projected image.
Brightness-reducing overfill of light valves, such as MDDs is a common problem that prior workers have toiled to solve. For example, U.S. Pat. No. 5,159,485 for SYSTEM AND METHOD FOR UNIFORMITY OF ILLUMINATION FOR TUNGSTEN LIGHT describes employing a tungsten lamp and an anamorphic optical system to illuminate an elongated, linear MDD array used for line-scanning a photo-sensitive drum in a printer. (Conventional enamorphic optical systems employ a lens or lenses having different focal lengths or magnification factors in perpendicular planes to the optical axis.) The anamorphic optical system receives a substantially rectangular light bundle from the tungsten lamp and squashes the light bundle in one axis so that the resulting squashed light bundle illuminates the elongated, linear MDD without substantial overfill. Unfortunately, employing a tungsten lamp without an integrator tunnel results in insufficient illumination uniformity and brightness for use in a multimedia projector.
What is needed, therefore, is an improved way of capturing as much of the light propagated through a color modulator as possible and uniformly imaging the light on a MDD without significant overfill.